Conventional stretch blow molding machines have an elongated blow core that is inserted a short distance into the open mouth of a preform at the blow station for the purpose of then injecting a surge of high pressure air into the warmed and softened body of the preform to stretch it quickly into the recesses of the mold cavity. A stretch rod normally associated with the core also moves through a quick stretching stroke to mechanically engage and stretch the body of the preform in cooperation with the air charge during the blow cycle. The blow core and stretch rod are then withdrawn and the finished product is removed from the blow station.
It is important to have a tight seal between the exterior of the blow core and the surrounding neck of the preform during the blow cycle, or else untoward air leakage will occur around the core as the high pressure air is injected. Leakage not only produces significant inefficiencies, but also may result in inadequate and incomplete stretching of all regions of the preform into some of the more distant and intricate recesses of the blow cavity.
Consequently, one conventional practice is to make the outer diameter of the blow core substantially the same size as the inner diameter of the preform neck. While this is helpful insofar as the leakage problem is concerned, it also increases the risk that the core will abrade or tear the inside surface of the neck during either insertion or withdrawal of the core. This can cause unsightly scuffing, marring, tearing, shearing or stress cracking of the neck, all of which is highly undesirable.
Another approach is to make the blow core slightly oversized relative to the neck. The tight fit which results is effective in preventing leakage, but in some instances the fit has been so tight that the core sticks in the neck and the machine is not powerful enough to raise the core up out of the preform when the blow cycle is finished.
One effort to solve the problem has been through the use of one or more sealing O-rings around the core, which is otherwise slightly undersized relative to the neck. However, the repeated abrading action of the O-ring against the neck surface during insertion and removal of the core may cause shards and slivers of the O-ring to fall into the container, which is also highly undesirable.